Sometimes you must be able to analyse Qemu more in-depth than the log files allow to.
For this, one can attach a <code>gdb</code> instance to a running Qemu process if it is started with the <code>-s</code> option.== Using GDB ==
For this, one can attach a [https://www.gnu.org/software/gdb/ GDB] instance to a running Qemu process as long as it is started with the <code>-s</code> option. To pass the required option from libvirt down to Qemu, one can use an additional namespace to pass along options to Qemu as descripted described [http://wiki.libvirt.org/page/QEMUSwitchToLibvirt#-s in the libvirt wiki].
Simply replace at the beginning of the XML (for example when editing using <code>virsh edit</code>):
and then start the VM.
The [https://www.gnu.org/software/gdb/ GDB] session to attach to the process and continue to run it until you hit a bug:
<pre>
~ # gdb
GNU gdb (Gentoo 7.6.2 p1) 7.6.2
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-pc-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.gentoo.org/>.
(gdb) file /usr/bin/qemu-system-
/usr/bin/qemu-system-: No such file or directory.
(gdb) file /usr/bin/qemu-system-x86_64
Reading symbols from /usr/bin/qemu-system-x86_64...Reading symbols from /usr/lib64/debug/usr/bin/qemu-system-x86_64.debug...done.
done.
(gdb) target remote localhost:1234
Remote debugging using localhost:1234
Reading symbols from /lib64/ld-linux-x86-64.so.2...Reading symbols from /usr/lib64/debug/lib64/ld-2.17.so.debug...(no debugging symbols found)...done.
(no debugging symbols found)...done.
Loaded symbols for /lib64/ld-linux-x86-64.so.2
0xfffff8003d7ab25f in _start () from /lib64/ld-linux-x86-64.so.2
(gdb) c
Continuing.
</pre>
== Generate core dumps ==
<source lang='bash'>
mkdir -p /var/virtualization/core-dumps
chmod 1777 /var/virtualization/core-dumps
echo "/var/virtualization/core-dumps/core.%e.%p.%h.%t" > /proc/sys/kernel/core_pattern
ulimit -c unlimited
/etc/init.d/libvirtd reload
</source>
... after that, all newly started Qemu processes will generate a (unique) core dump file in <code>/var/virtualization/core-dumps</code> in case they crash
[[Category:Snippets]]